1. Field of the Invention
The present invention relates to an apparatus for corneal surgery to correct a refractive error by ablating corneal tissue with a laser beam, and more particularly to an apparatus suitable for astigmatic correction.
2. Description of Related Art
An apparatus for corneal surgery to correct an refractive error of an eyeball by ablating a corneal surface with an excimer laser is conventionally known.
However, astigmatic correction, especially myopic astigmatic correction, performed by using this kind of apparatus has a problem that is a hyperopic shift of spherical component often occurs after such a correction. To address this problem, two methods have been suggested: one is to minimize a size of ablation area in a meridian direction for correction so as to reduce affect of the hyperopic shift, and the other is to combine myopic astigmatic correction and hyperopic astigmatic correction in order to correct myopic astigmatism.
The latter method is to estimate a hyperopic shift of the spherical component which will be developed after the correction of the myopic astigmatism so as to combine ablation of each correction in consideration of the estimated hyperopic shift. For example, in the case where a hyperopic shift of 33% estimated, upon correcting simple myopic astigmatism of which correction amount is S=0 D, C=xe2x88x923.0 D and A=0xc2x0, 75% of the correction amount, that is C=xe2x88x922.25 D and A=0xc2x0, is removed by ablation for myopic astigmatic correction. As the result, the hyperopic shift of 0.75 D, which accounts for 33%, occurs and the corneal refractive power after the ablation will be S=+0.75 D, C=xe2x88x920.75 D and A=0xc2x0. Here, replacing the sign of the astigmatism with a plus sign, the residual correction amount will be S=0 D, C=+0.75 D and A=0xc2x0, which is equal to 25% of the correction amount to be corrected by ablation for hyperopic astigmatic correction.
Further, in the case of mixed astigmatism of which spherical equivalent is 0, 37% of the correction amount is achieved by myopic astigmatic correction and 63% is achieved by hyperopic astigmatic correction. For example, in the case where the correction amount is S=+2.0 D, C=xe2x88x924.0 D and A=0xc2x0 by performing correction for S=0 D, C=xe2x88x921.5 D and A=0xc2x0 which is equal to 37% of the correction amount, C=xe2x88x924.0D in this case, a hyperopic shift of 0.5 D which accounts for 33% occurs and the resulting refractive power will be S=+2.5 D, C=xe2x88x922.5 D and A=0xc2x0. Replacing the sign of the astigmatism with a plus sign, the residual correction amount will be S=0 D, C=+2.5 D and A=90xc2x0, which is equal to 63% of the correction amount to be corrected by hyperopic astigmatic correction.
In the former of the above methods, since the range (the size) where the correction is effected is narrow, there is a problem that the patient may experience halos or glare when, for example, the pupil has dilated at night.
The latter method has been suggested to address the problem arise in the former method. Yet, there is another problem that the hyper shift rate needs to be obtained as an empirical value to estimate hyper shift of the spherical component caused by myopic astigmatic correction. In addition, this method has been applied only for correction of a certain limited range.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide an apparatus for corneal surgery which eliminates the need to obtain the hyper shift rate caused by astigmatic correction as an empirical value, and which can reduce adverse effect of change in the spherical component without a specific limitation on a corrective range upon astigmatic correction.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, an apparatus for corneal surgery to correct a refractive error by ablating corneal tissue with a laser beam emitted from a laser source and delivered onto a cornea of a patient""s eye with a light delivering optical system, the apparatus comprises irradiation area limiting means for limiting an irradiation area of the laser beam and for varying the irradiation area, first control means for controlling the irradiation area limiting means so as to reduce an ablation amount as the laser beam irradiates further away from a flattest meridian of astigmatism whereby effecting astigmatic correction, second control means for controlling the irradiation area limiting means so as to increase an ablation amount as the laser beam irradiates further away from a steepest meridian of astigmatism whereby effecting astigmatic correction, and arithmetic means for dividing a refractive power required for astigmatic correction into halves approximately equally so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means.
In another aspect of the present invention, an apparatus for corneal surgery to correct a refractive error by ablating an optical zone of a cornea with a laser beam, the apparatus comprises input means utilized for inputting each data necessary for correction, an optical system, first control means for controlling the optical system so as to bring a longitudinal axis of the slit aperture into coincidence with a flattest meridian of astigmatism whereby gradually changing the slit width of the slit aperture so that the laser beam that passes through the circular aperture and the slit aperture ablates more amount as it is closer to the flattest meridian and less amount as it is farther from the flattest meridian, second control mean for controlling the optical system in a manner that a longitudinal axis of the slit-like laser beam in a rectangular shape, which is limited by the circular aperture, is made parallel to a steepest meridian of astigmatism whereby gradually changing an eccentricity amount of the laser beam so that the laser beam that passes through the circular aperture and the slit aperture ablates less amount as it is closer to the steepest meridian and more amount as it is farther from the steepest meridian, and arithmetic means for dividing amount of astigmatic correction into halves approximately equally on the basis of the inputted data so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means. The optical system includes a circular aperture of which opening diameter is variable, a slit aperture of which slit width is variable, a projection lens for projecting the apertures onto the cornea, a moving unit for making an area irradiated by the laser beam eccentric with respect to a center of the optical zone, and a rotator for rotating the laser beam.
Further, in another aspect of the present invention, an apparatus for corneal surgery apparatus to correct a refractive error by ablating an optical zone os a cornea with a laser beam, the apparatus comprises input means utilized for inputting each data necessary for correction, an optical system, first control means for controlling the optical system in a manner that the scan mirror makes the laser beam in the rectangular shape scan an area limited by the circular aperture having a diameter larger than that of the optical zone and by the slit aperture of which longitudinal axis is brought into coincidence with a flattest meridian of astigmatism whereby gradually changing the slit width of the slit aperture after each scan, second control means for controlling the optical system in a manner that a longitudinal axis of the laser beam in the rectangular shape, which is limited by the circular aperture having a diameter larger than that of the optical zone, is made parallel to a steepest meridian of astigmatism whereby gradually changing an eccentricity amount of the laser beam with respect to the steepest meridian by the scan mirror so that the laser beam ablates more amount as it is farther from the steepest meridian, and arithmetic means for dividing amount of astigmatic correction into halves approximately equally on the basis of the inputted data so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means. The optical system includes a circular aperture of which opening diameter is variable, a slit aperture of which slit width is variable, a projection lens for projecting the apertures onto the cornea, a scan mirror for making the laser beam in a rectangular shape in a manner that it crosses the opening of the circular aperture or slit aperture, and an image rotator for rotating the laser beam on an optical axis of the optical system.